What is a Qubit? – A bit of Information on the Qubit!


You may have heard of quantum computing; but you may not yet have heard of the fundamental building block of information that make up quantum computational operations. In order to explain what this qubit is, it is important to explain the classical bit which you may already be familiar with.

Imagine you are trying to answer a question systematically. One which only has two possible answers (for example: a yes or no question). It would follow that you need two types of output states for this system to occupy which when measured, indicate those two answers. Imagine one were to label this overall output state “C”. Classically “C” can take the value of yes or no. For this example we will label yes as ∣1⟩, and no as ∣0⟩. This means that in order to explain to someone the properties of an unknown and unmeasured state “C”, one would need to explain that “C” is equal to ∣0⟩ or ∣1⟩. This is a classical bit of information. You may impart the answer to one question with two possible answers using an output state “C”.

After understanding the classical bit, one may more easily understand the qubit. Like the classical bit, the qubit can be used to answer a question that has two possible answers (∣0⟩ or ∣1⟩). However, while this remains true, the qubit does not only have only two output states. It has infinite! While this may not be immediately obvious, it can be explained.

This is because, when one measures the output state of a qubit (which we will label ψ); one would still only measure one of the two possible answers (∣0⟩ or ∣1⟩). However if you were to repeat this measurement under the same discernible starting conditions you may get an unexpected result! The output may be ∣0⟩ the first time and ∣1⟩ the second. If you were to then repeat these measurements many times you would eventually find that while it is impossible to predict the answer with certainty; you can expect the output ∣0⟩ a fixed proportion of the time, and you can expect an output ∣1⟩ another fixed proportion of the time. So the infinite output states are the different proportional likelihoods that a measure output will be ∣0⟩ or ∣1⟩.

Imagine an output state ψ. We will label the proportion of the time that ψ will be measured to be ∣0⟩: α. As the only other state of the qubit that could possibly be measured is ∣1⟩, we can label the probability of measuring ∣1⟩ to be 1-α. Therefore the difference between a bit and a qubit can be described as so:

For an unmeasured classical bit output state C:
C = ∣0⟩, or C = ∣1⟩

However for an unmeasured qubit output state ψ:
ψ = α⋅∣0⟩ + (1-α)⋅∣1⟩
Where α is a probability.

This was intended to be a brief explanation of the qubit. I hope it clicks!

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